1. Field of the Invention
This invention relates to a hot forming unit, also know as a debiteuse, for plate glass production, with a noble metal box that is encompassed by and contained in a highly heat-resistant ceramic frame and has a drawing slot that is matched to the cross section of a plate glass sheet to be produced.
2. Description of Related Art
In the drawing of plate glass in a Fourcault process, hot forming units called debiteuses are used, which are made of ceramic or other fireproof materials and float on the molten glass because their density is lower than the density of the glass to be drawn, the glass being drawn upward through the debiteuse. In the text that follows, the term ceramic includes all suitable fireproof materials.
Even with horizontal drawing of plate glass, debiteuses of this type are used, which form a component of the melting apparatus, such as the melting end or the channel that adjoins the melting end. The debiteuses are of one or more ceramic components with a horizontally disposed drawing slot. The debiteuses are subject to a large amount of wear. In order to achieve a high degree of dimensional integrity of the drawing slot and a sufficiently long service life of the debiteuse, the debiteuses have a noble metal lining, for example a platinum or a platinum alloy, so that the lining is the only thing contacting the glass flowing through and therefore is the glass contact material. Thus, the lining is of a box-shaped form, a noble metal frame, which is inserted into the drawing slot and can also be directly heated electrically in order to prevent the glass flow from cooling prematurely.
The debiteuses that are lined with a noble metal frame, however, have one disadvantage that the base material of the forming unit and the noble metal lining have different thermal expansion behaviors, such as the noble metal lining has a greater linear thermal expansion than the base material, for example ceramic, of the hot forming unit.
This is disadvantageous, particularly when heating the hot forming unit and when there are changes in the processing temperature of the glass to be drawn. The greater linear thermal expansion of the noble metal lining in relation to the ceramic of the base material causes folds to appear in the lining inside the drawing slot, which not only cause glass defects, but also cause damage to the noble metal lining and lead to a more intense corrosion by the glass flow. This shortens the service life of the hot forming unit. A further disadvantage lies in the fact that the known debiteuses have a temperature distribution that is unfavorable for the drawing process and that cannot be individually adjusted.